1. Field of the Invention
This invention relates to a process for preparing stable inorganic pigment the particles of which are coated with fine amorphous silica and which has good pigment characteristics, i.e. high chemical resistance, hydrogen sulfide resistance, light resistance, weatherability, heat resistance and storage stability.
2. Description of the Prior Art
Pigments are widely used as colorants for paints, printing ink, and many varied articles such as plastics, rubber, construction materials, cosmetics and paper. They are also used as an agent for providing protective coatings because of advantageous physical and chemical properties such as their hiding power, covering power, reflecting properties, insulating effect and rust preventing power. However, the pigments have undesired properties of their own, for example there is the chalking of titanium oxide, the heat- or light-induced discoloration of chrome yellow and yellow oxide and the decomposition of inorganic pigments (e.g., cadmium pigment, ultramarine, etc.) with acid. Hence, the type and amount of pigments to be used as essentially limited by the manufacturing process and the use of the articles to which the pigments are applied. On the other hand, the type of pigments determines what articles they should be applied to. In addition, with the recent improvement in the technology of forming and processing olefinic plastics as well as the use of forming temperatures higher than 250.degree. C., the demand for highly heat resistant pigments has increased. Under these circumstances, various industrial sectors have expressed a strong need for the improvement in pigment characteristics such as light resistance, weatherability, chemical resistance, hydrogen sulfide resistance, heat resistance, storage stability and dispersibility. It is true that many processes for improving the characteristics of pigments have been proposed and implemented in commercial plants, but the fact is they have their own advantages and disadvantages. For example, Japanese Patent Publication No. 16531/74 teaches a method of covering pigment particles with a metal oxide by hydrothermal treatment with heat and pressure, but the method does not fully provide the desired improvement in resistance probably because the particles of the metal oxide grow during the hydrothermal treatment. The method described in Japanese Patent Publication No. 9555/71 covers the particles of a lead chrome pigment with the fine silica produced from a dilute aqueous solution of sodium silica and dilute sulfuric acid. The resulting silica cover makes the pigment very resistant in various aspects, but because of the strong acidity of sulfuric acid, the method has the potential hazard of damaging the pigment and therefore, cannot be used in general applications. It has been confirmed that the fine silica sol produced by decomposing an aqueous solution of alkali silicate using specific compounds is very active and has an extremely high capability for covering the particles of pigment. Therefore, a technique that can cover the particles of whatever type of pigment with such silica sol would be a great contribution to the pigment industry where the demand for improved pigment characteristics is strong. However, it is considerably difficult to cover the particles of various pigments with such reactive silica sol of minimum particle size without adversely affecting the particles. Some combinations of pigment and alkali silicate decomposer may damage the pigment. The slightest variation in the temperature and pH used in decomposition of the aqueous sodium silicate results in a considerable change in the covering power of the silica and, particularly, the choice of the decomposer which is reacted with the aqueous silicate is the most predominant of the factors that affect the covering power of the silica.
As a result of various studies on a method of covering the particles of a pigment with a protective coat which is free from the defects described above, the inventors have found that certain acid gases will react with an aqueous solution of alkali silicate and form an extremely fine and active silica which will not damage the particles of inorganic pigment. The inventors have also found that the pigment the particles of which are coated with such silica has remarkably high resistance.